The development of a real-time non-invasive method to measure the multiple components of blood flow would be enormously useful for a number of cardiovascular, neurological, renal, and radiological applications. Currently, the only means of obtaining multiple blood velocity components is through MRI phase velocity mapping techniques, which are cumbersome, time-consuming, and limited in temporal resolution. Furthermore, MRI phase velocity mapping and its various counterparts are not real-time techniques. We have recently developed an ultrasound-based particle image velocimetry technique that has shown promise in the measurement of complex blood flow patterns. This method, termed echo-PIV, takes advantage of the non-linear backscatter characteristics of ultrasound contrast micro bubbles analyzed in the RF domain to distinguish individual micro bubbles, which are then tracked over time to obtain the local velocity vector. We have developed software algorithms to analyze this information and have obtained promising data from in vitro and in vivo studies. The method now requires optimization and implementation into hardware to form a real-time non-invasive velocimetry method. Our experience with ultrasound and Doppler imaging, experimental fluid dynamics including the development and use of a variety of optical PIV systems, and long-standing relationship with the ultrasound industry, make the satisfactory completion of this project highly probable. The specific aims of this project are: 1) Use numerical modeling of backscatter from micro bubbles to study which driving conditions (frequency, pulse shape, pulse length, power, etc.) will maximize the non-linear response of the bubbles and thereby increase the accuracy of the echo PIV algorithm. 2) Assemble the hardware components to implement the real-time echo PIV imaging system, based on maximizing the specifications for the two clinical applications described above. 3) Test the prototype imaging system using in vitro models with laser-based optical particle image velocimetry as the standard for comparison.